Determining the displacement of objects, in particular objects consisting of at least one layer of sheetlike material, such as loose sheets of paper, stacks of paper, signatures, or envelopes, constitutes an important part of the operation of various office machines, including machines for composing items to be mailed. Determining the displacement of documents serves, for instance, to determine the position of a mark on a passing document relative to a reference mark or relative to the leading edge of that passing document. Another exemplary application is the measurement of the length of a sheet, a stack of sheets or an envelope by determining the displacement between the passage of the leading edge and the trailing edge. Still another exemplary application is to stop a sheet, a stack of sheets or an envelope with the leading edge, the trailing edge or a particular mark in a particular position. Such an application forms part of a method for assembling sheets of different lengths into a stack, as described in applicant's European patent application 0,556,922, which corresponds with U.S. patent application Ser. No. 08/019,431.
From German patent application 2,300,421, it is known to follow the displacement of a sheet by having a pulse disc move along with the displacement of the sheet.
In U.S. Pat. No. 5,138,640 it is discussed that the resolution of a system with a pulse disc could be refined by increasing the number of pulse producers circumferentially distributed over the disc. However, this entails a drawback in that a correspondingly large number of pulsed signals are generated, which signals must be processed with priority. However, processing these pulsed signals with priority requires a powerful processing system because such processing takes up a considerable part of the system's capacity, which therefore is not available for other functions. In practice, this means that a powerful microprocessor or separate hardware would be necessary for registering the angular displacement.
In this U.S. patent specification it is further discussed that the resolution of a system with a pulse disc can be increased by interpolation between successive pulses. One of the discussed ways of achieving this is based on the determination of the time between successive pulses. According to another method discussed, clock signals between successive pulses are counted and the angular displacement of the pulse disc following a pulse is partly determined on the basis of the quotient of the number of clock signals following that pulse and the number of clock signals per pulse. According to that patent specification, a more accurate determination of the angular displacement of the pulse disc at varying speeds can be achieved by using two clock signals, the frequency of a second clock signal being n times the frequency of a first clock signal. The number of pulses of the second clock signal per interpolation pulse is set to be equal to the number of pulses of the first clock signal between two pulses of the pulse disc. As a result, the number of interpolation pulses per pulse of the pulse disc in principle equals n and this number returns to n after any deviations by speed variation.
These interpolation methods also entail the drawback that they require a relatively large processing capacity because the interpolation pulses constitute additional signals that are to be processed with priority so as to limit inaccuracies resulting from variations in the processing time of the interpolation pulses.